The invention relates to a phase-locked loop comprising a voltage-controlled oscillator to which a control voltage is applied as produced by a phase/frequency detector as a function of the difference between the frequency of a reference signal and the output frequency of the voltage controlled oscillator, the oscillator containing as a frequency-influencing circuit element a varactor whose capacitance value can be varied over a fine adjustment range by the control voltage for altering the output frequency. The invention relates furthermore to a method for automatically setting the output frequency of one such phase-locked loop.
In conventional phase-locked loops the components of the voltage-controlled oscillator dictating the frequency are selected so that the output frequency of this oscillator can be maintained at a desired setpoint value within a fine adjustment range to thus make it possible to compensate deviations in the tolerance of the capacitance value of the varactor employed as the component dictating the frequency by the phase/frequency detector outputting a control voltage which changes the capacitance value so that the capacitance value of the varactor needed to achieve the desired output frequency is set. This results in the output frequency of the voltage-controlled oscillator remaining constant as long as the circuit experiences no changes due to temperature, aging and/or production which necessitate a change in frequency exceeding the fine adjustment range. To produce large frequency changes in the output frequency of the oscillator the components dictating the frequency in the voltage-controlled oscillator need to be replaced so that the oscillator can operate at the desired new setpoint frequency.
The invention is based on the objective of configuring a phase-locked loop of the aforementioned kind so that the circuit automatically adjusts to the new desired output frequency even when large changes in the frequency are wanted.
This objective is achieved in accordance with the invention by providing a variable capacitance which can be connected in parallel to the varactor when there is a change in the frequency of the reference signal and by the value of this capacitance being adjustable as a function of the control voltage output by the phase/frequency detector.
This configuration of the phase-locked loop in accordance with the invention permits automatically adapting the frequency-dictating component in the voltage-controlled oscillator to the newly desired output frequency which now permits the phase-locked loop to be put to use in a wide frequency range and, more particularly, also permits it being achieved in an integrated circuit.